Hybrid Time-power Switching Protocol of Energy Harvesting Bidirectional Relaying Network : Throughput and Ergodic Capacity Analysis

In this paper, we investigate system performance in term of throughput and ergodic capacity of the hybrid time-power switching protocol of energy harvesting bidirectional relaying network. In the first stage, the analytical expression of the system throughput and ergodic capacity of the model system is proposed and derived. In this analysis, both delay-limited and delay-tolerant transmission modes are presented and considered. After that, the effect of various system parameters on the proposed system is investigated and demonstrated by Monte-Carlo simulation. Finally, the results show that the analytical mathematical and simulated results match for all possible parameter values for both schemes.


Introduction
Energy harvesting (EH) wireless communication has been attracting intensive research interests from both academia and industry, as it can substantially prolong the network lifetime, especially for wireless sensor networks with low power nodes.Energy can be harvested from environment resources, but EH from conventional sources, such as solar and wind, may be sporadic or intermittent, which makes it difficult to satisfy the data transmission requirements.One particular energy source is the radio frequency (RF) signal, which can recharge nodes more controllable.Wireless energy transfer (WET) in the network with a helping relay is the most popular technique.
The helping relay with energy harvesting has been proposing and studied in many last papers.In more details, [3] presented energy harvesting in amplify-and-forward (AF) relaying.In [4], the total energy harvested from multiple sources was optimally allocated among different destinations.In [5], the effect of large-scale network interference on energy harvesting decodeand-forward (DF) was considered.Moreover, the effect of the random location of the relay on DF relaying is investigated in [6] and [7] maximized the achievable throughput of an AF energy harvesting system.Furthermore, a similar problem was studied in [8] for DF and [9] proposed the achievable throughput of an AF energy harvesting system was optimized.From this point of view, energy harvesting relaying network is the hot direction in the communication network and is necessary to develop more and more.
In this work, the hybrid time-power switching protocols for the energy harvesting bidirectional relaying network is presented.In this relaying network, the outage probability and the ergodic capacity are proposed and demonstrated for both delay-limited (DL) and delay-tolerant (DT) transmission modes.The main contributions of the paper are summarized as follows: a.The system model of the hybrid time-power switching protocols for the energy harvesting bidirectional relaying network is proposed for both delay-limited (DL) and delay-tolerant (DT) transmission modes.b.The closed-form of the outage probability and ergodic capacity for the system is derived.c.The influence of the main parameters on the system performance is demonstrated entirely.
The structure of this paper is proposed as follows.The system model of the hybrid time-power switching protocols for the energy harvesting bidirectional relaying network is proposed and presented in section 2. Sections 3 proposed outage probability and ergodic 1948 capacity for both delay-limited (DL) and delay-tolerant (DT) transmission modes are investigated and derived, respectively.Section 4 provides the numerical results and some discussions.Finally, some conclusions are proposed in section 5.

System Model
In this section, the system model is presented in Figure 1.In this model, the user is intended to send information to the access point (AP) with the assistance of a relay R and the AP transfer the energy to the user by the helping relay.Moreover, the direct connection between the AP and the user is so weak, hence, the only available communication path as well as power transfer path is via the relay R. The relay R plays both roles of energy relaying from the AP to the user and information forwarding from the user to the AP [10,11].All nodes are assumed to operate in half-duplex mode, and decode-and-forward (DF) can be used at the relay for information transfer.Regarding the channel model, the perfect channel state information (CSI) is available at the relay and the AP.All channels here experience Rayleigh fading and keep constant during each transmission block so that they can be considered as slow fading.
The Figure 2 displayed the time-power switching based protocol model.In the first interval time αT, the AP transfer energy by RF to the relay R, 0≤α≤1.After that, the energy transfer from the AP to the R and the information transmission from the U to the R are performed instantaneously in the second interval βT, 0≤β≤1.Moreover, finally, the information transmission process from R to AP and the energy transfer process from the R to U are made in the remaining interval time (1-α-β)T.In this model, we assume that the circuit power consumption is negligible as compared to the radiation power, which is reasonable for lowpower devices such as sensor nodes.In this system model, 0≤α≤1 and 0≤β≤1.If α=0, this scheme becomes a power splitting protocol.If β=(1−α)/2 and ρ=0 then it becomes the time switching protocol.

The outage probability and ergodic capacity
In this part, the system performance of the hybrid time-power switching protocols for the energy harvesting bidirectional relaying network is presented, analyzed and demonstrated for both delay-limited (DL) and delay-tolerant (DT) transmission modes in details in this section.Let x u denote the transmitted signal from the user during the second phase, and P u denotes the power of this signal.Then, the received signal at the relay R can be calculated by: After pre-canceling the signal x ap the remaining part of the received signal at the relay is written as Where n r ∼N(0,N 0 ) denotes the Gaussian distributed noise at the relay R and then the received signal at the user in the third interval is expressed as: Where P u : the power at the user, P r : the power at the relay, n r ,n d : the additive white Gaussian noise (AWGN) at R, U with zero mean and variance N 0 , h and g he channels from the AP to the relay and from the user to the relay, respectively.The harvesting energy E r at R in the first and second interval time can be formulated as: From that harvesting energy, the transmission power of the R in the thirst interval can be calculated as the following: ap ap r r ap

P h T P h T E P kP h TT
Where we denote () . Similarly, while the AP receives the information signal, the mobile user also receives the RF energy sent from the relay.By ignoring the noise energy, which is negligible as compared to the signal energy.The received signal at the mobile user is y u =gx r .Hence, the energy harvested during this phase can be determined by: So the transmit power of the user during the thirst phase is expressed in the below equation:

22
(1 ) 1 In this case, we select ( ) 1,   because P r , P u must be positive.

The delay-limited transmission mode (DL)
From ( 5), the signal to noise ratio (SNR) at the relay R can be computed by the following equation: Where we denote .Similarity, the signal to noise ratio (SRN) at the user based on (8) can be formulated by: (1 ) Where we denote Proof 1 See Appendix A.

The Delay-Tolerant transmission mode (DT)
In this section, the ergodic capacity from the source to relay   22 22 , log ( 1) From the equations ( 11) and ( 12), the ergodic capacity of the proposed system can be chosen as: Then the throughput of the proposed system can be calculated by the below equation: ( Theorem 2. The exact integral forms for the ergodic capacity r C and u C of the proposed system can be expressed as the followings: Proof 2 See Appendix B.

Numerical Results and Discussion
In this paper, the Monte Carlo simulation was conducted to verify the analysis developed in the previous section.For simplicity, in our simulation model, we assume that the source-relay and relay-destination distances are both normalized to unit value.For the delaylimited transmission mode, the outage probability, and achievable throughput are analyzed in details.On the other hand, the outage probability, and the ergodic capacity for the delay-tolerant transmission mode are proposed and demonstrated.
Figures 3 and 4 plot the effect of ρ on the outage probability and system throughput in the DL transmission mode, respectively.In this simulation, we set P ap /N 0 =10 dB and α, β in Figures 3 and 4.Moreover, Figures 5 and 6 present the dependent of the outage probability and system throughput in the DL transmission mode on the η of the proposed system.Furthermore, Figures 7 and 8 show the influence of the outage probability and system throughput in the DL transmission mode on the ratio P ap /N 0 .From the results, we can see that the analytical and simulation results well agree with each other.On another way, Figure 9 and Figure 10 plot the effect of ρ on the outage probability and system throughput in the DT transmission mode, respectively.In this simulation, we set P ap /N 0 =10 dB and α, β in Figures 9 and 10.Moreover, Figures 11 and 12 present the dependent of the outage probability and system throughput in the DT transmission mode on the η of the proposed system.Furthermore, Figures 13 and 14 show the influence of the outage probability and system throughput in the DT transmission mode on the ratio P ap /N 0 .From the results, we can see that the analytical and simulation results well agree with each other.Also, the Figure 15 presents Comparison of the throughput versus P ap /N 0 for DT and DL transmission modes for the both analytical and simulation cases.

Conclusion
In this paper, the hybrid time-power switching protocol of energy harvesting bidirectional relaying network is proposed and investigated.In order to analyze the system performance, analytical expressions for the outage probability, ergodic capacity and the throughput of the delay-limited and delay-tolerant transmission mode are investigated ad derived.The results show that the analytical mathematical and simulated results by Monte-Carlo simulation match for all possible parameter values for both schemes.The results could be provide the prospective solution for the communication network in the near future.In this case, to compute the system throughput, we will evaluate the outage probability and the throughput of the proposed system by the followings: By changing the variable The equation ( 14) can be obtained by substituting (A7) to (A5). is the complete gamma function which is defined in [12].Then we can rewrite as:

Figure 1 .
Figure 1.System model Figure 2. The hybrid time-power switching based protocol model Hybrid Time-power Switching Protocol of Energy Harvesting ... (Tran Hoang Quang Minh) 1949

rCC
, and for relay to the user u are formulated.By using the received signal SNR in (8),(9), the ergodic capacity

Figure 3 .
Figure 3.The outage probability versus ρ for DL transmission mode